Water oxidation photocatalysis : Mn-based co-catalysts on TaON

TaON was studied as a photocatalytic material for visible light driven water oxidation. Anomalies in the structure of the material such as lattice strain or oxygen deficiencies were shown to have a negative influence on its photocatalytic activity, as defects act as recombination centers for the photogenerated charge carriers. TaON was loaded with a manganese-based co-catalyst to enhance its activity. Mn deposition was realized following two procedures: deposition-precipitation and impregnation. The loading procedure was shown to have a great influence on the morphology and dispersion of the co-catalyst and therefore on its activity. Having showed more promising results, we deepened our investigations on the deposition-precipitation procedure and studied the influence of the pH-control agent on the morphology of the sample. To measure electrochemical and photocatalytical OER-activity we adapted to our system two testing methods based on the use of two sacrificial agents: the CAN test and the Ag test

Frustration driven structural distortion in VOMoO4

Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetization measurements and electronic structure calculations in VOMoO4 are presented. It is found that VOMoO4 is a frustrated two-dimensional antiferromagnet on a square lattice with competing exchange interactions along the side J1 and the diagonal J2 of the square. From magnetization measurements J1+J2 is estimated around 155 K, in satisfactory agreement with the values derived from electronic structure calculations. Around 100 K a structural distortion, possibly driven by the frustration, is evidenced. This distortion induces significant modifications in the NMR and EPR spectra which can be accounted for by valence fluctuations. The analysis of the spectra suggests that the size of the domains where the lattice is distorted progressively grows as the temperature approaches the transition to the magnetic ground state at Tc=42 K

Influence de la pluviométrie sur la contamination de l'atmosphère et des eaux de pluie par les pesticides

Cette étude a pour objectif d'identifier les facteurs qui influencent la contamination des eaux de pluie par les produits phytosanitaires. Cinq sites contrastés ont été choisis de manière à être représentatifs des zones de productions légumières ou de plein champs et à couvrir les différents modes de contamination des précipitations. Il s'agit des sites de l'Ile de Ouessant, Landivisiau, Plouay, Ploufragan et Rennes. Les évènements pluvieux collectés sont choisis en fonction des caractéristiques de formation de la perturbation et du calendrier des épandages de pesticides.Par ailleurs, les concentrations rencontrées pour le site de Rennes en 2000 (année très humide) ont pu être comparées à celles obtenues lors d'une étude conduite en 1996 sur un site proche mais pour des conditions climatiques plus habituelles (année humide à sèche).Les analyses sont réalisées par extraction en phase solide suivie d'une analyse en chromatographie en phase gazeuse couplée à la spectrométrie de masse, ou par détection azote-phosphore spécifique (NPD) ou détection par capture d'électrons pour les composés halogénés (ECD). Dans ces deux derniers cas, la confirmation de l'identité des produits est réalisée par un système de double colonnes.Sept évènements pluvieux distincts ont été collectés et analysés entre les 15 mars et 15 juillet de l'année 2000. Parmis les produits recherchés six molécules sont régulièrement retrouvées : l'atrazine et son métabolite la déethylatrazine (DEA), l'alachlore, le lindane (gamma HCH) et son isomère le béta HCH ainsi que la desméthryne. Nous avions déjà des observations analogues à l'issu de nos premières investigations de 1996.Les analyses réalisées ont montré l'existence d'une contamination chronique du compartiment atmosphérique par l'atrazine et la DEA pour les zones sous influence agricole. Les niveaux de concentrations rencontrés sont faibles, de l'ordre de 10 ng/L. A ce bruit de fond s'ajoutent en période de traitement, des transferts depuis les parcelles traitées qui conduisent à des valeurs beaucoup plus élevées (de 0,1 à 0,7 µg.L-1). La détection de l'alachlore et de la desmethryne est limitée aux périodes d'application de ces produits.Du lindane, et dans un cas son isomère le béta-HCH, ont pu être mis en évidence sur quelques prélèvements, traduisant une contamination chronique du compartiment aérien due à la rémanence de ce type de composés.Les concentrations dans les pluies, pour un site donné, sont très dépendantes de la pluviométrie. Alors qu'en année de pluviosité normale (1996), les concentrations en période d'application sont élevées, elles restent faibles pour une année humide (2000). Pour s'affranchir de l'effet de dilution, nous avons calculé des retombées massiques (mg.ha-1) pour les deux périodes de collecte de 1996 et 2000. Les résultats de 2000 restent malgré cela très inférieurs à ceux de 1996 (d'un facteur quatre environ). Les surfaces emblavées et les différences d'usage entre les deux années sont trop faibles pour expliquer les écarts obtenus. Ceci nous permet de conclure que c'est l'intensité des transferts sol-air qui détermine les niveaux de contamination de l'atmosphère. L'humidité élevée des sols, pour une année humide, favorise la migration verticale des produits phytosanitaires dans les couches inférieures, réduisant ainsi les concentrations de pesticides susceptibles d'être transférés vers le compartiment aérien.Contamination of rainwater by pesticides was investigated by analyzing samples from 5 locations in Brittany (France). These sites were chosen on the basis of various characteristics including agricultural practices around the site, the direction of prevailing winds (west to east) and the proximity to heavily populated areas. The sites chosen were: Ile de Ouessant (a natural reserve without pesticide application), Landivisiau (a semi-natural zone), Plouay (a rural site with intensive agriculture, mainly cereals and vegetables), Ploufragan (a suburban site) and Rennes (an urban area surrounded by intensive agriculture and orchards). The pesticides to be monitored included the compounds most commonly used in Brittany. New molecules were added to this list to take into account new homologues or uses. Some organochlorines and pesticides with specific application modes were thus included in the list. Analytical techniques involved solid-liquid extraction procedures followed by GC-NPD, GC-ECD or GC-MS after prior derivatisation. Derivatisation was performed with heptafluorobutyric acid to detect urea and sulfonylurea herbicides. Quantification limits were about 5 ng/L for GC-ECD or GC-NPD, and 50 ng/L for molecules detected by GC-MS after derivatisation.Rain episodes were selected according to the prevailing meteorological conditions and the air mass trajectory. Only precipitation events greater than 6 mm were considered, in order to collect sufficient volume for analyses. Thirty-five samples (7 per location) were collected between 15 March and 15 July 2000, corresponding to various weather fronts.Six pesticides were detected: atrazine and its metabolite de-ethylatrazine (DEA), alachlor, desmethryne, lindane and its isomer (beta HCH). Among those detected most concentrations were below 100 ng/L but higher concentrations were occasionally detected during and after spraying in agricultural areas. Lindane was detected at low concentration (<10 ng/L) in several samples collected during different periods and locations. These results demonstrate chronic atmospheric contamination due to the presence of this substance even though its use has been banned in the Economic European Community (EEC) (e.g. in 1999 in France).The most frequently detected compound was atrazine, detected in 60 % of the samples. The presence of atrazine and its metabolite DEA in rainfall indicate chronic contamination of the atmosphere outside of spraying time (mid-March to mid-April). Detection of these compounds was generally limited to agricultural sites, and could also indicate transfer from agricultural areas during soil preparation for maize sowing. These hypotheses will be tested by monitoring atrazine in rainwater during a complete agricultural cycle. Important contamination by atrazine was detected during spraying time at agricultural locations, where concentrations ranged from 10 to 60 ng/L. Detection of alachlor was limited to application periods and rural sites, at levels ranging from 20 to 240 ng/L. However, amounts of alachlor were also detected in Ouessant, even though its use is banned on this isle. This presence was presumably due to the specific conditions of formation of the weather fronts that resulted in rainfall.Concentrations of pesticides in rainwater obtained in 2000 were generally lower than those described in literature or in a previous study done in Brittany. Alachlor and atrazine concentrations were compared to those of a previous study conducted in Brittany in 1996. The two sites chosen were Rennes in 2000 and Le Rheu in 1996. These two locations, separated by 5 km, were not significantly different since Rennes is directly downwind of Le Rheu. Pluviometry was very important in the year 2000, showing a 30 % increase in rainfall compared to the average from 1961-1990. The concentrations of pesticides in both years were also very different. During a year of normal rainfall (e.g., 1996) concentrations of pesticides in application periods were high, but they remained low during wet years (e.g., 2000). These differences could not be explained by a dilution effect - when fall-out was calculated (mg/ha) to correct for the dilution effect, the differences remained important. Maize surfaces (132 000 ha in 1996 compared to 128 000 ha in 2000) and the quantities of pesticides applied (580 tonnes in 1994 in Brittany compared to 350 tonnes for atrazine, and 263 tonnes in 1998 compared to 119 tonnes in 1994 for alachlor) were too similar to explain the differences observed in 1998. Photodegradation processes, which are more important during normal or dry years, should also favour higher concentrations in 2000 than in 1996. Rain concentrations and fallout observed in 2000 remained lower than those obtained in 1996. Thus, it can be concluded that atmospheric contamination is lower for wet years (e.g., 2000) than for normal or dry years (e.g., 1996). Soil humidity, high for wet years, favours the vertical migration of these substances to lower soil layers, reducing pesticide concentrations available to be transferred to the atmosphere

Dietary fibre enrichment of supplemental feed modulates the development of the intestinal tract in suckling piglets

Background: Commercial pre-weaning diets are formulated to be highly digestible and nutrient-dense and contain low levels of dietary fibre. In contrast, pigs in a natural setting are manipulating fibre-rich plant material from a young age. Moreover, dietary fibre affects gastrointestinal tract (GIT) development and health in older pigs. We hypothesised that supplemental diets that contain vegetal fibres are accelerating GIT development in suckling piglets in terms of size and functionality. From d 2 of life, sow-suckled piglets had access to a low fibre diet (CON), a diet with a fermentable long-chain arabinoxylan (lc-AXOS), a diet with a largely non-fermentable purified cellulose (CELL), or a diet containing both fibres. During the initial 2 weeks, the control diet was a high-density milk replacer, followed by a dry and highly digestible creep meal. Upon weaning at 25 d, 15 piglets from each treatment group, identified as eaters and originating from six or seven litters, were sacrificed for post-mortem examination of GIT morphology, small intestinal permeability and metabolic profile of the digesta. The microbiota composition of the mid-colon was evaluated in a sub-set of ten piglets. Results: No major statistical interactions between the fibre sources were observed. Piglets consumed the fibre-containing milk supplements and creep diets well. Stomach size and small intestinal permeability was not affected. Large intestinal fill was increased with lc-AXOS only, while relative large intestinal weight was increased with both fibre sources (P < 0.050). Also, CELL decreased ileal pH and tended to increase ileal DM content compared to CON (P < 0.050). Moreover, the concentration of volatile fatty acids was increased in the caecum (P < 0.100) and mid-colon (P < 0.050) by addition of CELL. lc-AXOS only stimulated caecal propionate (P < 0.050). The microbiota composition showed a high individual variation and limited dietary impact. Nonetheless, CELL induced minor shifts in specific genera, with notable reductions of Escherichia-Shigella. Conclusions: Adding dietary fibres to the supplemental diet of suckling piglets altered large intestinal morphology but not small intestinal permeability. Moreover, dietary fibre showed effects on fermentation and modest changes of microbial populations in the hindgut, with more prominent effects from the low-fermentable cellulose

Evidence for an unconventional magnetic instability in the spin-tetrahedra system Cu_2Te_2O_5Br_2

Thermodynamic experiments as well as Raman scattering have been used to study the magnetic instabilities in the spin-tetrahedra systems Cu_2Te_2O_5X_2, X=Cl and Br. While the phase transition observed in the Cl system at T_o=18.2 K is consistent with 3D AF ordering, the phase transition at T_o=11.3 K in the Br system has several unusual features. We propose an explanation in terms of weakly coupled tetrahedra with a singlet-triplet gap and low lying singlets.Comment: 4 pages, 4 figure

Impact of Periodic Current Pulses on Li-Ion Battery Performance

International audiencePulse charging and pulse discharging have been reported by many authors in the literature to improve the performance of various secondary electrochemical cells. Only a few authors mentioned the effects of such charge and discharge method on lithium-ion batteries. The overall objective of this work is to experimentally investigate the impact of certain current pulse profiles on the electrical performance of Li-ion batteries. The results highlight a detrimental impact of periodic pulses on the cell performance compared to profiles with constant current